Here are some tips to consider when measuring your or your partner's finger:

After sliding the ring sizer on, make sure it slides off easily over the knuckle.
Don't measure cold fingers, as this is when fingers are their smallest.
For the most accurate reading, measure the finger at the end of the day when the finger is the largest.

If you're buying the ring as a surprise,

Ask your partner's mother or one of their friends if they know their finger size.
Secretly, take one of your partner's rings and get it measured at a jewellery shop.

You can call Customer Service on (0800) 093 7436 for a Free Ring Sizer

Find a piece of string or strip of paper.
Wrap it around the base of your appropriate finger. Make sure the paper fits snugly. You should be able to slip it over your knuckle.
Using a pen - mark the point on the string/paper where it overlaps forming a complete circle.
Measure the length from the starting end of the string/paper to the pen mark using a ruler with a millimetre measurement.
Check this measurement with the chart below to determine your ring size.

Important:

To get the most accurate reading, make sure your hands and fingers are warm, preferably at room temperature.